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The S-400 order is seen as one of the key symbols of NATO member Turkey's flourishing relationship with Russia under Erdogan and Putin.

The order has raised questions among NATO allies over the systems' compatibility with NATO equipment and also the broader implications for Turkey's future strategic orientation.

President Donald Trump in August signed a defence authorisation act that could prohibit the delivery of F-35 stealth aircraft to Turkey if it goes ahead with the purchase of the S-400, which can shoot down missiles as well as aircraft.

According to Russian media, Turkey is buying four S-400 systems in a contract worth a total of USD 2.5 billion.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)